Centrifugal pumps generally comprise a driven impeller which rotates within a pump casing or body. The impeller generally carries a number of blades or vanes which extend outwardly from the axis of rotation and which serve to force the fluid drawn into the impeller outwardly into the casing or pump body, thereby creating a suction at the central area which serves to draw fluid along an axial direction into the inlet of the impeller and delivering the fluid to the outlet at a higher peripheral velocity. Generally, the pump casing or body includes a volute, diffuser, or other system for converting the higher velocity fluid into a pressure head.
In many situations, particularly with respect to centrifugal pumps which are designed to operate within a wide range of capacities, the fluid being pumped is subjected to cavitation because of the system where the pump is included or by virtue of, for example, operating at part load conditions. In other situations, such as, for example, when a centrifugal pump is operated at reduced flows, strong internal recirculation can be generated. Both cavitation and internal recirculation result in fluid pressure pulsations which frequently can lead to rapid destruction or, at the very least, extensive wear of the various component parts of the pump, such as, for example, pump bearings, the impeller shaft and/or impeller vanes, thus limiting the useful life of the pump. Generally, the undesirable effects of cavitation and/or internal recirculation at part load operations are more pronounced in larger centrifugal pumps, and the wear and/or damage owing to such conditions can increase with increasing rotational speeds of the impeller.
Numerous efforts have been made to reduce the wear or damage attributable to such problems. Some of the more common attempted solutions have included limiting the interval of operation of the pumps in part load regions, utilizing highly wear-resistant materials for those parts which are most likely to be affected and/or providing inserts which are installed in regions where the effect of cavitation and/or internal recirculation are most likely to induce rapid wear or destruction and to provide for replacement of such inserts at regular intervals or when the need arises.
Another attempted solution for minimizing or reducing the effects of cavitation-induced erosion is disclosed in U.S. Pat. No. 4,239,453. In accordance with this patent, the centrifugal pump is provided with an annular diffuser element installed upstream of the intake or inlet to the impeller. The diffuser has a relatively small cross-sectional flow area remote from the intake to the impeller, which progressively increases in cross-sectional flow area towards the inlet to the impeller. The purpose for this arrangement, as disclosed in the patent, is to reduce the effect of eddy currents created at reduced rates of flow, which the patentee thereof believed to be one of the major causes of cavitation.
Impellers used in centrifugal pumps are generally of two types--namely, open-faced and closed. In open-faced types of impeller designs, the edges of the vanes or blades which are remote from the inlet to the impeller are secured to a shroud plate or rear disc member so that the vanes extend substantially at right angles to the surface thereof towards the inlet to the impeller. An impeller of the closed type includes an additional shroud plate or disc member secured to the edges of the vanes nearest the inlet of the impeller so as to provide an impeller chamber bounded by the two spaced shroud plates, with the vanes dividing the impeller chamber into subchambers. The second shroud plate or disc member includes a central aperature in register with the inlet port of the pump to permit fluid entering therethrough to enter between the two shroud plates and to then be thrown outwardly upon rotation of the impeller and the vanes associated therewith. Examples of closed-type impeller constructions are shown in U.S. Pat. Nos. 2,882,829 and 4,556,364, as well as U.S. Pat. No. 4,239,453.
In such prior art closed impeller constructions, both of the shroud plates or disc members are secured to the edges of the vanes throughout substantially their entire radial extent. It will thus be appreciated that with such closed impeller constructions, there is no cross-communication of fluid between the various subchambers within the immpeller chamber, i.e. fluid introduced into any of the subchambers remains in such subchamber until it is expelled from the impeller. Thus, in such closed impeller constructions, fluid pressure pulsations, due to the effects of cavitation, reduced flow or as a result of other conditions, act upon a substantially rigid impeller structure. Consequently, the effects of pressure and flow surges can often be more pronounced in such closed impeller structures.